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UEPARFMEFT OF THE IMIERIOF
<br />UNREO STATES OEOfLU1CAL SURVEY
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<br />TO ACCaMPANY MAP I-Q!
<br />GEOLOGY OF THE SEDIME*ITARY ROCKS OF THE
<br />MORRISON QUADRANGLE, COLORADO
<br />By 7. Hiram Smith
<br />The Morrison quadrangle is in Jefferson County,
<br />about l0 miles west of Denver, Colo, The rugged west-
<br />ern one-third of the quadrangle along the eastern flank
<br />of the Front Range is underlain by Precambrian crys-
<br />talline rocks. The eastern two-thirds, with which this
<br />account is concerned, lies in the Colorado Piedmont
<br />section of the Great Plains and is underlain by sedi-
<br />mentary rocks, Prominent topographic features on
<br />the plains include (I) the long sinuous hogback formed
<br />by the Dakota Group; (2)South Table Mountain,abtoad
<br />relatively flat mesa at the notch edge of [he quadrangle;
<br />(3) Green Mountain, a broad gently rounded conspicuous
<br />eminence in the east-central par[ of the quadrangle;
<br />and (4) Mt. Carbon, aflat-topped remnant of a middle
<br />Pleistocene pediment south of Bear Creek. Theprin-
<br />cipal streams-Clear Creek, which crosses the north-
<br />west corner of [he quadrangle, and Mount Vernon and
<br />Bear Creeks-have cut deep canyons in the Front
<br />Range, Clear Creek and Bear Creek flow eastward
<br />from the mountain front across the sedimentary rocks
<br />of [he Piedmont and into the South Platte River, but
<br />Mount Vernon Creek is diverted by the hogback formed
<br />by the Dakota and empties into Bear Creek at Morri-
<br />son,
<br />The author has benefited from many discussions
<br />with Richard Van Horn. Dr, H. D. MacGini[ie of Hum-
<br />boldt Sate College contributed helpful suggestions con-
<br />cerning [he Cretaceous-Paleocene boundary andiden-
<br />tified fossil leaves from the Denver and Green Moun-
<br />tain Formations, The Cretaceous invertebrate fossils
<br />were identified by W, A. Cobban, and the zones of [he
<br />Pierre Shale were mapped by G, R, Scott, Scott and
<br />Cobban collected most of the fossils by which the zones
<br />of the Pierre Shale were mapped,
<br />Geologic observations in this area were made as
<br />early ae 1869 by [he Hayden Survey, but the earliest
<br />comprehensive geologic study was made by Emmons,
<br />Cross, and Eldridge (1896). More recentstudies were
<br />made by Johnson (1931) on paleontology, Covering and
<br />Goddard (1938) and Waldschmidt (1939) on the Table
<br />Mountain Glows, LeRoy (1946) and Van Horn (1957) on
<br />general geology, Brown (1943) and Reichert (1954) on
<br />the Tertiary sedimentary rocks, WaagE (1955) on the
<br />Dakota Group, Scott and Cobban (1959) on the Pierre
<br />Shale, Maughan and Wilson (1960) and Huber[ (1960)
<br />on [he Fountain and Lyons Formations, and Berg(1962)
<br />on the Golden fault.
<br />The bedrock of the Mor rison q u a d r a n g l e com-
<br />prises Precambrian metamorphic and intrusive rocks,
<br />which were no[ mapped, sedimentary rocks of Paleo-
<br />zoic and Mesozoic age, and sedimentary rocks and
<br />interbedded volcanic Flows of early Cenozoic age, These
<br />rocks are mantled by alluvium and colluvium of ~~uater-
<br />nary age, The sedimentary rocks consist of IS for-
<br />mations that range in age from Pennsylvanian toTer-
<br />tiary (table 1) and have a combined thickness ~~f about
<br />13,000 feet, The contact of the Precambrian t:rystal-
<br />line rocks with the overlying Fountain Formation is
<br />sharply unconformable, and locally the crystalline
<br />rocks beneath it are deeply weathered.
<br />Paleozoic sedimentary rocks.--The Pale rzoic is
<br />represented by the Fountain Formation, Lyor,s Sand-
<br />stone, and part of the Lykins Formation. Th ~ Foun-
<br />tain and Lyons are resistant ro erosion and fo-m bold
<br />outcrops. The contact between them is gradational,
<br />the characteristic lithologies of the two for nations
<br />inn•rfingering within a 30-foot transition zone. The
<br />Lylyitls Formation was subdivided by LeRoy (1946)
<br />mto three shale members separated by two thin lime-
<br />stone members. The uppermost or Strain Shale Mem-
<br />ber constitutes about two-thirds of [he formation; its
<br />upper part is of Triassic(7) age. Most of the Lykins
<br />is not resistant to erosion, and, except for the 3lennon
<br />Limestone Member, forms a broad valley. TlteClen-
<br />non forms a narrow sharp hogback, The baeat contact
<br />of the Lykins probably is disconformable, Tht Lykins
<br />is the brightest red bed in the quadrangle.
<br />Mesozoic sedimentary rocks.--Rocks ofTAesozo-
<br />ic age include those from the upper par[of the Lykins
<br />Formation to the lower part of the Denver Formation.
<br />The lower contact of the Ralston Creek Formation is
<br />not exposed but is disconformable in adJoinini; areas.
<br />The disconformity marks a hiatus encompassing Early,
<br />Middle, and part of Late Jurassic time, The Ralston
<br />Creek locally Corms a low ridge. The Morrison For-
<br />mation formsasteep smooth slope interrupted byemall
<br />ledges of sandstone and limestone, A basal sandstone
<br />of the Morrison locally lies in channels in the Ralston
<br />Creek Formation and indicates a minor uncom'ormity.
<br />Near Morrison, the formation has yielded many fossil
<br />bones of giant dinosaurs (Marsh, 1877),
<br />The Dakota Group was divided by Waag2 (1955,
<br />p. 18-19) into the Lytle and South Pla[[e Formations,
<br />and the South Platte was subdivided into five members.
<br />Although all these units are recognizable locally in
<br />the Morrison quadrangle, they were not mapf~ed sep-
<br />arately, Almost the entire group ie exposed along the
<br />Alameda Parkway, but elsewhere expoeures are• incom-
<br />plete. The basal ron[act is disconformable, th? lower-
<br />most conglomerate beds of the Lytle lying in channels
<br />cut into the Morrison, Sandstone units in [he• Dakota
<br />Group form a prominent hogback,
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